Supporting Tauranga City Council’s Integrated Stormwater Project

This paper, Supporting Tauranga City Council’s Integrated Stormwater Project, was presented at Water New Zealand’s 2017 Stormwater Conference. By Carl Johnson (DHI, Auckland).

ABSTRACT A large storm event occurred in Tauranga in April 2013. Consequently, Tauranga City Council (TCC) came under significant pressure to remedy the flood hazards apparent to the public.

The first step in the flood mitigation efforts that were part of TCC’s Integrated Stormwater Project (ISP) was to develop computer-based models of all catchments across the city for flood-hazard assessment and mitigation-option testing.

DHI was engaged by TCC to assist in the field of stormwater modelling.

At the beginning of the ISP, initiated in 2013, TCC had seven flood models already constructed by a number of different consultants based on a variety of approaches.

As part of the ISP, four engineering consultants were selected to construct the 12 remaining stormwater catchment models. A significant effort was made to standardise the model-build approach, to ensure some degree of consistency between models.

DHI provided simulation technology, produced technical components of the tender briefs, undertook the technical peer review, and provided software and flood-modelling assistance to both TCC and the consultants.

Technical aspects that set the models constructed for TCC apart from previous studies include: explicit representation of all council-owned sumps, rain-on-grid hydrological approach, and the combination of raised footprints and high roughness to represent building blockage.

Each of these approaches is justified by practical model-build experience and the intended use of the results.

A wide range of documents has been produced to support the ISP, including flood-modelling guidelines, largely-standardised technical tender briefs, a tested peer-review schedule, peer-review checklists and peer review reporting.

As a result of the ISP, TCC has moved from providing an indicative level of service (too costly) to focusing on flooding that directly threatens life.

Many of the flood models, developed as part of the ISP, have already been used successfully in options assessments.

CONCLUSIONS

1) Even relatively small flood events, when compared to those that are used in design scenarios, can have large political impacts for councils.

2) The flood modelling component of TCC’s ISP has been a success. The models produced are of a high and consistent level of quality. Flood maps are available on the council’s website and some of the models are already being used in options analysis.

3) Flood model builds take much longer than estimated, often for reasons out of control of the consultant. In general, it is unlikely that a model can be completed within a year’s period. The longer a model takes to build, the greater the chance that staffing of the project team will change.

4) It is possible to include a high level of detail in flood models without significant increases in model build effort. Indeed, with the right procedures in place data entry effort and reliance on on-the-spot decision making can be reduced. Flood models will become more data-intensive in the future.

5) Maintaining a consistent technical methodology in projects such as the ISP is difficult because of the number of people involved from all parties (many being non-technical), the multi-year duration, and attending staff movements. This problem is exacerbated by the difficulty of handing over models of this complexity. By and large, these issues have been successfully managed by the various techniques outlined in this paper.

6) The relatively small pool of flood modellers in New Zealand means that consultancies struggle to maintain modelling teams and experience, so clients must be aware that the success of a modelling project often comes down to the individuals involved and not the reputation of the consultancy.

7) In order to maintain consistency across models, the peer review process was extensive and fine-grained, almost to the point where the reviewer was part of the model-build team.

8) A large body of documentation resulted from the ISP: this is necessary to protect TCC’s investment in the flood models.